MNT: Refactors the code to adopt flake8

[Gui-FernandesBR]

Pull request type

• Other (please describe): Linters!

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

Pylint is the only linter adopted in our repo

New behavior

Flake8 is a simpler and faster linter that can be used to identify potential coding or formatting errors in a python file.

I added flake8 check to the CI right before the pylint checker.
A few errors pointed out by flake8 were not being detected by pylint, which really illustrates the importance of having at least 2 different linters running on production.

Breaking change

• No

Additional information

• I tested ruff as well, but it sounds too "experimental" to me. Maybe a few months we can rediscuss it.
• Use make format to run isort and black.
• Use make lint to run flake8 and pylint

Please run these commands before pushing to origin, the reviews will be much easier now.

[Gui-FernandesBR]

Flake8 is really faster! It will help us quite a lot.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.92%. Comparing base (6908414) to head (fd4941c).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #631   +/-   ##
========================================
  Coverage    73.91%   73.92%           
========================================
  Files           70       70           
  Lines        10032    10032           
========================================
+ Hits          7415     7416    +1     
+ Misses        2617     2616    -1     

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